1. Field of the Invention
The present invention relates to a strand filling compound for electrical cables, and cables including such a compound, which prevents migration of water lengthwise of the stranded wires of the conductor of the cable.
2. Description of the Prior Art
It is known in the art that cable insulation is deteriorated by the development and propagation of water/chemical trees when moisture is present in regions of the insulation structure of the cable and particularly in regions of localized high electrical stress caused by voids, contaminants and protrusions from the conductor and insulation stress control layers. Water present in the spaces between the strands of a multi-stranded conductor is recognized as significantly accelerating the propagation of water/chemical trees in the insulation. It is therefore desirable to fill all spaces between such wires with a filling compound so as to minimize or prevent ingress and movement of water in such spaces. See, for example, U.S. Pat. Nos. 5,049,593; 4,095,039; 4,145,567; and 3,943,271.
As pointed out in U.S. Pat. No.4,095,039, some of the prior art filling compounds are not satisfactory after aging due to loss of adhesive qualities and hardening and fracturing thereof. The patent discloses the use of low molecular weight polyisobutylene rubber or a low molecular weight copolymer of isobutylene-isoprene rubber with 40-150 parts by weight of electrically conductive carbon black or graphite added thereto as a filling compound, such compound having a putty-like consistency at room temperature and good adhesiveness which is retained during operation of the cable.
Another advantage indicated by the use of the filling compound disclosed in U.S. Pat. No. 4,095,039 is that the filling compound can be applied by means of a pump supplying the compound to an applicator where the spaces between the wires are filled and the wires coated. This indicates that the compound is flowable and does not require high pressures for its application. This property eliminates the need for an extrusion head and the expense thereof which is necessary for some filling compounds which can be extruded over the wires.
U.S. Pat. No. 4,145,567 discloses a similar filling compound made of ethylene propylene rubber compounded with a substantial amount of carbon black so as to give it a putty-like consistency and a drip-point above 100.degree. C.
It has been found that the filling compounds of said U.S. Pat. Nos. 4,095,039 and 4,145,567 are quite effective as filling compounds for stranded conductors of electrical cables, but it has also been found that some problems still exist from the standpoint of preventing ingress and flow of water in the conductor while preventing overfill of the spaces under the outer layer of wires. Thus it has been found that under some conditions, such as cable handling, subsequent manufacturing operations and after accelerated cyclic aging test simulating cyclic loading of the cable in the field some ingress of water into the conductor and movement over a limited length of the conductor may occur although a significant improvement has been achieved.
Furthermore, in the manufacturing operation of completely filling the spaces between the strands, the volume of filling compound is difficult to control and the extent of the fill can vary along the length of the stranded conductor. In this regard it should be recognized that normally the outer interstices of the stranded conductor are filled by the pressure extrusion of the conductor stress control layer and no supplemental filling of the spaces in required. Where the spaces between the outer layer of wires and the next to outer layer of wires is overfilled with filling compound, the extrusion of the semiconducting stress control layer over the stranded conductor can cause penetration of filling compound into the outer interstices of the conductor resulting in protrusion of the extruded semiconducting stress control layer into the insulation. Since such protrusions result in localized high electrical stresses in the insulation, such effects are highly undesirable. In some cases, it is considered desirable to also fill the outer interstices of the conductor with filling compound in which case a semiconducting rubber filled tape is applied lapped on itself over the completely filled conductor followed by extrusion of the semiconducting stress control layer. In this case overfilling of the spaces between the outer layer of wires and the next to outer layer of wires is not objectionable and does not represent a problem.
U.S. Pat. No. 5,049,593, which is incorporated herein by reference, has solved the foregoing problems by providing an improved filling compound of the type disclosed in said U.S. Pat. Nos. 4,095,039 and 4,145,567. The '593 patent teaches a filling compound comprising a low molecular weight rubber admixed with fine particles of a material and/or with a thin layer of fine particles of material applied over the filling compound which swells when it absorbs water. The filling compound of the '593 compound is provided with 15 to 150 parts by weight of the electrical conductive filler to 100 parts of the low molecular weight rubber (isobutylene rubber) compound and preferably from 15 to 50 parts to impart electrical conductivity to the compound. 5 While the strand filling compound of the '593 patent is very effective, there are some drawbacks which have heretofore been unsolved in the art. Furnace grade carbon black which is the most commonly used conductive filler in prior art strand filling compounds, is known to be a source of ionic contamination in the insulation system. However, furnace grade carbon black is the least expensive conductive filler suitable for strand filling compounds. Therefore, while it is known to use "cleaner" conductive fillers in order to reduce ionic contaminants, in practice, the use of cleaner conductive fillers is cost prohibitive.
Strand filling compounds for use in electrical conductors generally must be pumpable at elevated temperatures about 100.degree. C. or greater; must not drip or flow out of the conductor stands at temperatures at least up to 130.degree. C.; must be compatible with the conductor shield as specified in ICEA Publications T-32-645-1993 "Guide for Establishing Compatibility of Sealed Conductor Filler Compounds with Conducting Stress Control Materials", and T-25-425-1981" Guide for Establishing Stability of Volume Resistivity for Conducting Polymeric Components of Power Cables"; and must pass a water penetration test in accordance with ICEA Publication T-34-664-1996 "Guide for Conducting Longitudinal Water Penetration Resistance Tests on Longitudinal Water Blocked Cables".
It is therefore an object of the present invention to provide a strand filling compound which may be designed with a cleaner conductive filler, such that ionic contamination of the insulation system is minimized, without incurring a cost penalty for the strand filling compound.
It is further an object of the present invention to provide a low cost strand filling compound with the same superior performance as found in the art.